Ocean modifier with volume
From Haunt's trick bag:



&nbsp;&nbsp;Since the Ocean modifier doesn't use vertex groups, one solution is to get help from a modifier that does. The Ocean modifier (gray) uses an ordinary plane. The block of water is a mesh where the upper surface is highly subdivided and the bottom contains just 4 verts. It has a Vertex Group which excludes the four bottom verts.

&nbsp;&nbsp;The trick is a Shrinkwrap Modifier which is projecting the surface on the Ocean Modifier mesh. There the Vertex Group helps to leave the bottom verts in peace.



&nbsp;&nbsp;Rendering of the Ocean Modifier is prevented in the outliner by clicking the camera symbol.